During manufacture or flat paper and plastics products, various sheet properties of multi-layered and single layer sheets can be detected with visible and infrared radiation while the sheet-making machine is operating. Characteristics of the sheet including composition, basis weight, coating weight, moisture content, opacity and layer thicknesses can be measured by sensors which detect the amount of radiation that the sheets absorb, transmit or reflect from a beam of infrared light or other radiation. A typical sensor includes an infrared (IR) radiation source that directs a beam of IR radiation towards a sample and the beam is transmitted through beam conditioning optics, such as collimating lenses and/or focusing lenses. These lenses condition the optical radiation for optimal sensor efficiency. The optics in front of the detectors typically comprises focusing lenses and those adjacent to the sample are typically collimating or focusing lenses. IR radiation is partly absorbed, reflected and transmitted by the sample depending on its various properties. A beam splitter splits the IR radiation into two separate beams with each beam being directed to separate band pass filters that are positioned and aligned immediately before detectors. The hand pass filters are configured to pass IR radiation at selected regions of the infrared spectrum. IR radiation, which is not within the selected region of the spectrum, is reflected by the filters back to the beam splitter. Adsorption-type filters can be used although they are less efficiency that the band pass filters which are interference-type filters. Instead of employing a beam splitter which requires a multiplexing arrangement, the sensor can use a rotating filter-wheel assembly. For example, a circular array of filters rotating around a shall or pivot is positioned to the side of the optical path defined by IR radiation reflected from the sample such that a circle drawn through the centers of the filters passes through the center of the optical path. As the filter-wheel rotates, different filters are introduced into and removed from the optical path.
Depending on the intensity of the radiation detected, the detector generates an analog electrical signal that may be converted to a digital signal for observation. The described sensor arrangement can measure different properties of the sample under observation. For instance, in the thickness measurement of thin plastic films, one of the two infrared band pass filters only passes infrared radiation having wavelengths in a selected region of the infrared spectrum. This first region of the spectrum is called the “reference” region, and the associated detector is called the “reference” detector. The reference channel spectral range is located in a specific region of the IR spectrum, which is not associated with a signature absorption band of the material or materials, which the film is composed of. This reference channel however should be indicative of all other optical loss mechanisms in the sensor system and sheet that are not indicative of the optical absorption of the material being sensed. These other properties may include such things as scattering loss from the sheet or the insertion losses of the optical components used.
Similarly, in papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, gloss, and sheet caliper. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process.
Generally, on-line measurements of sheet properties are made by scanning sensors that travel back and forth across the width of the sheet in the cross-machine direction (CD). In the manufacturing of a flat sheet of paper, the cross-machine direction uniformity is a critical issue. The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices or profile zones.
The sensors include a radiation source that typically comprises a broadband infrared source and a receiver with one or more detectors with the wavelength of interest being selected by narrow-band filters such as, for example, an interference type filter. The sensor gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (typically called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.